With the advance of display systems illumination technology from incandescent to fluorescent to solid-state light sources, and with ever-increasing miniaturization, one popular electronic category seems not to have kept pace. That category is large-sized personal data displays, such as personal computer monitors.
For many years, such monitors were based on cathode ray tube (“CRT”) technology. More recently, flat panel displays have increasingly displaced CRT displays. The most common form of flat panel displays utilizes one or more fluorescent light sources located behind a liquid crystal display (“LCD”) screen. Contemporary technology has enabled the use of cold cathode fluorescent light (“CCFL”) light sources, but because a cathode emitter is still required, a high voltage source for striking and maintaining an electric arc through the CCFL is also required.
With continuing improvements in light-emitting diode (“LED”) technology, such as substantial improvements in brightness, energy efficiency, color range, life expectancy, durability, robustness, and continual reductions in cost, LEDs have increasingly been of interest for superseding CCFLs in larger computer displays. Indeed, LEDs have already been widely adopted as the preferred light source in smaller display devices, such as those found on portable cellular telephones, personal data assistants (“PDAs”), personal music devices (such as Apple Inc.'s iPod®), and so forth.
One reason for preferring LED light sources to CCFL backlight light sources is the substantially larger color gamma that can be provided by LED light sources. Typically, an LCD display that is illuminated by a CCFL backlight produces about 72-74 percent of the color gamma of a CRT-based NTSC display. (“NTSC” is the analog television system in use in Canada, Japan, South Korea, the Philippines, the United States, and some other countries.) Current LED backlight display technology, however, has the potential of producing 104-118 percent or more of that gamma color space.
Another reason for not preferring CCFL bulbs is that they contain environmentally unfriendly mercury, which could be advantageously eliminated if an acceptable LED backlight light source configuration could be developed for larger displays.
When implemented in small displays such as just described, the technical requirements are readily met. As is known in the art, the illumination intensity can be rendered uniform by distributing LED light sources around the periphery of the display and utilizing light diffusing layers behind the display to equalize the display intensity. The technical challenges are modest because the screens are modest in size, so that the individual display pixels are never very far from one or more of the LED light sources. Light attenuation caused by distance from the LED light sources is therefore not great and is readily equalized by appropriate LED positioning coupled with suitable light diffusers behind the display.
One way to envision the ease with which this challenge can be met in smaller displays is to consider the number of pixels, on average, that each LED light source must support in the display, and the maximum distances per pixel that the most distant pixels are located relative to a given LED light source. These numbers are modest (perhaps in the hundreds), so the light diminution or attenuation for the most distant pixels is similarly modest and readily compensated by suitable diffuser designs.
On the other hand, the larger geometries of typical flat panel computer monitors and displays (e.g., larger than about 20 inches) create area-to-perimeter ratios that have proven untenable for current LED technologies, particularly with respect to LED brightness or light output. This has meant that it has proven unsatisfactory to attempt to replace CCFL light sources with LED light sources along one or more edges of such larger display screens. Accordingly, such displays continue to employ CCFL light sources even though CCFL light sources are increasingly less desirable than LED light sources.
It would seem that a straightforward solution for replacing CCFL light sources with LEDs would then be to arrange the LEDs in some sort of array configuration behind the LCD display screen, rather than around the perimeter. Prior attempts to do so, however, have proven unsatisfactory. Commercially viable displays for general consumption must be economical to manufacture, thin, lightweight, must provide efficient thermal management capability, and must provide consistent and uniform color quality and brightness throughout the display, all at reasonable costs. Attempts to meet these criteria in acceptable form factors and costs have been unsuccessful.
Previous efforts to achieve these objectives have failed due to a number of practical obstacles. For example, even though LED light outputs have dramatically improved in recent years, a very large number of LEDs is still required to provide sufficient brightness in such larger displays. Typically, a minimum of several hundred LEDs must be used. This then requires an enormously large maze of wires and/or bulky circuit boards to mount, support, and power such a large number of LEDs in a distributed matrix configuration. This in turn requires adequate mechanical structure to support all those components behind the LED screen. The resulting structure is bulky, thick, heavy, and not well suited for managing and removing the heat that is generated by the LEDs and the underlying electrical circuitry. It is also expensive and not well suited for efficient manufacturing.
Another challenge with utilizing LEDs in large arrays is maintaining uniformity of color in the large numbers of LEDs. The color balance and spectra of the LEDs is limited by the phosphorescence. For example, white LEDs are often actually blue LEDs with a complementary phosphor dot on the front of the LED. Depending upon manufacturing precision (and thus, related manufacturing costs), actual colors may vary from, for example, slightly blue to slightly pink. Understandably, reducing or compensating for such variability increases cost and complexity significantly as the number of LEDs increases in larger display configurations and environments.
The color and the output of each LED also depend fairly sensitively on temperature. The difficulties in providing proper thermal management capability can readily lead to temperature variations across the distributed array of LED light sources. Since the color qualities of LED light sources are sensitively dependent upon their operating temperatures, such non-uniformities lead to unacceptable variations in color from one portion of the display to another.
Another major obstacle to commercialization of such larger LED light source displays is the complexity and costs associated with measuring and calibrating each such display as it is being manufactured. Prior CCFL displays commonly use one, or at most just a few, CCFL light sources, so the necessary calibrations and corrections, such as color correction and gamma correction, can be easily accomplished and managed. For example, a single CCFL light source will provide uniform and homogeneous color and gamma for the entire display, so localized corrections are not usually a concern. The need for highly customized color corrections for individual displays has also been basically eliminated due to quality control advances in CCFL light source technology that has led to economical production of CCFL light sources that consistently provide reliably uniform illumination profiles.
Such is not the case with LED light source displays that include multiple LED light sources distributed at various display locations. When employed in larger displays, as previously described, the LEDs may be distributed throughout the area behind the display, and not just along the perimeter edges. This results in possible performance variations that can result from any number of causes, for example, temperature variations from one region of the display to another.
Calibration of a display may be accomplished by adjusting the imaging layer, such as a display's thin film transistor liquid crystal display (“TFT-LCD”). Calibration of the TFT-LCD to compensate for LED variability can be complex due, among other reasons, to the properties of the TFT-LCD itself. For example, there can be cross talk between color channels due to interaction properties of the LCD elements. Other calibration adjustments may be required due to non-linearities of output with brightness, asymmetrical RGB (“red, green, and blue”) transfer functions for the color channels, differing gamma profiles, proper and accurate gray tracking, and so forth.
As a result, it has been important to measure and calibrate each LED light source display to establish profiles for each such display that enable compensations to be made for those intrinsic factors. Compensations can be made, for example, by appropriately changing the image renditions formed by the TFT-LCD panel of the display to reverse and neutralize the LED performance variations. The compensations can be managed, for example, by the device that controls the display (e.g., a computer) or by suitable circuitry within the display itself. However, each display must first be appropriately measured and carefully calibrated. Heretofore this has been a time-consuming and expensive process, acceptable perhaps for limited-production, “high-end” specialty displays, but not acceptable for mass-produced consumer products.
As a result, prior efforts to replace CCFL light sources with LEDs in commercial consumer applications have largely failed to move beyond the prototype stage. The complexities, manufacturing costs, bulkiness, very heavy weights, color non-uniformities, thermal management challenges, calibration complexities and costs, and so forth, have simply combined in such a way as to leave experts in the technology convinced that they must yet await the development of even significantly brighter, more uniform, and less expensive LEDs.
Consumers expect and demand an excellent, consistent, and affordable consumer experience. Prior attempts to utilize LEDs in large displays have thus not solved the problem of building displays that are light, easy and inexpensive to manufacture, uniform in color, low in cost, and that also provide the excellent overall high quality user experience that customers demand and expect.
Thus, a need still remains for improved display color-correcting systems, and especially for improved gray tracking that is provided efficiently, effectively, and economically, with reduced measurement sets. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations, and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to save costs, improve production and product efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.